Various rubbers are often mixed with curvatives such as sulfur and/or organic peroxides in order that they can under go vulcanization or curing. It is recognized that, if desired, various vulcanization accelerators can also be mixed with the rubber in order to speed up the curing of the rubber or, in other words, shorten the vulcanization time.
Some uncured rubber materials, or compounded rubber, can be used for injection molding various articles with suitable injection molding equipment. Such articles can include, for example, ring-shaped tire treads for the purpose of retreading worn tires, solid tires, flaps for tube type truck tires, motor mounts for various mounts and tank track pads.
Vulcanization, or curing, relates to a process where elastomers, natural and/or synthetic, are mixed with various materials which will cause the rubber to undergo a type of crosslinking effect upon application of heat. Various other materials are conventionally mixed with the rubber to help improve various properties such as strength, durability and the like. Such rubber mixture is often called compounded rubber. In general, the vulcanization step will effect various changes in raw, uncured elastomers such as increasing its tensile strength, eliminate tack, increase elasticity, elongation and hardness.
As hereinbelow set forth, the enhanced properties can be obtained by mixing the uncured elastomer with a curative such as, for example, sulfur in the presence of various accelerators. Conventionally, the presence of accelerators in the vulcanization process is desirable because they enable the vulcanization reaction to be carried out over a shorter period of time and, often, at a lower temperature.
The use of various vulcanization accelerators can present particular problems in the preparation of various molded articles. For example, accelerators can rapidly accelerate the vulcanization at conventional rubber processing temperatures so that the uncured elastomer can become partially or prematurely cured before the molding and curing step. The resultant "pre-partially vulcanized" rubber is somtimes termed "scorched rubber". In order to overcome this disadvantage, some types of accelerators are used which are known as "delayed action accelerators" in which their accelerator effect becomes evident only after the elastomer reaches an elevated tempeature range. Such accelerators thus tend to reduce or eliminate the aforesaid scorching problems. Additionally, a prevulcanization inhibitor may further be used to delay premature onset of vulcanization in processing especially through the injection molding step.
Even the use of such delayed action accelerators and prevulcanization inhibitors may not be sufficient to effectively reduce premature curing of elastomers for some purposes such as, for example, in some instances of injection molding operations.
Accordingly, advantageous rubber compounding technology is often sought for various purposes, such as for example, the preparation of rubber compounds which might be used for injection molding purposes.
When an unvulcanized elastomer is submitted to injection molding procedures, sufficient pressures are applied by suitable equipment to force the elastomer into a mold cavity with sufficient pressure to cause the elastomer to fill the mold. The elastomer composition is submitted to curing conditions in the mold such as elevated temperature and pressure in order to create a vulcanized article.
In the operation of the injection molding process, a strip of raw stock is fed into a heated cylinder where it is put under pressure by either a reciprocating ram or helical screw. The helical screw units masticate the stock as well as heat it, then move forward like a ram when the mold is charged. During the charging of the mold, the ram or ram screw forces the heated stock out of the cylinder through a nozzle into runners in the mold and through sprues into the cavities. The nozzle then disengages until the cure is complete and the mold is emptied and reclosed.
In the injection molding process, the stock has to travel a long distance through the runners and sprues in the mold. In these runners and sprues, the rubber compound is submitted to relatively high shear forces which cause substantial friction. This internal friction rapidly increases the temperature of the rubber compound. Very often the cure of the rubber starts before the compound reaches the mold. In such a case, the cured rubber clogs the runners and sprues and the injection process is interrupted. Due to the high heat generation of the stock, lower amounts of sulfur and accelerators have to be used. Due to the lower amount of curatives, the compound will, therefore, vulcanize at a slower rate. It is, therefore, desirable to overcome such injection molding difficulties for the purpose of injection molding of rubber to form molded articles therefrom.